With continued regulatory pressure there is a growing need to identify more environmentally sustainable replacements for refrigerants, heat transfer fluids, foam blowing agents, solvents, and aerosols with lower ozone depleting and global warming potentials. Chlorofluorocarbon (CFC) and hydrochlorofluorocarbons (HCFC), widely used for these applications, are ozone depleting substances and are being phased out in accordance with guidelines of the Montreal Protocol. Hydrofluorocarbons (HFC) are a leading replacement for CFCs and HCFCs in many applications; though they are deemed “friendly” to the ozone layer they still generally possess high global warming potentials. One new class of compounds that has been identified to replace ozone depleting or high global warming substances are halogenated olefins, such as hydrofluoroolefins (HFO) and hydrochlorofluoroolefins (HCFO). In the present invention, it was discovered that chloro-trifluoropropenes are particularly useful refrigerants liquid chiller systems, particularly in negative-pressure chiller systems, such as for the replacement of R-11 and R-123.
With continued regulatory pressure there is a growing need to identify more environmentally sustainable replacements for refrigerants, heat transfer fluids, foam blowing agents, solvents, and aerosols with lower ozone depleting and global warming potentials. Chlorofluorocarbon (CFC) and hydrochlorofluorocarbons (HCFC), widely used for these applications, are ozone depleting substances and are being phased out in accordance with guidelines of the Montreal Protocol. Hydrofluorocarbons (HFC) are a leading replacement for CFCs and HCFCs in many applications; though they are deemed “friendly” to the ozone layer they still generally possess high global warming potentials. One new class of compounds that has been identified to replace ozone depleting or high global warming substances are halogenated olefins, such as hydrofluoroolefins (HFO) and hydrochlorofluoroolefins (HCFO). The HFOs and HCFOs provide the low global warming potential and zero or near zero ozone depletion properties desired.
Chillers are refrigeration machines that cool water, other heat transfer fluids, or process fluids by a vapor-compression (modified reverse-Rankine), absorption, or other thermodynamic cycle. Their most common use is in central systems to air condition large office, commercial, medical, entertainment, residential high-rise, and similar buildings or clusters of buildings. Both large central and interconnected plants, generally with multiple chillers in each, are common for shopping centers, university, medical, and office campuses; military installations; and district cooling systems. The chilled water (or less commonly a brine or other heat-transfer fluid) is piped through the building or buildings to other devices, such as zoned air handlers, that use the cooled water or brine to air condition (cool and dehumidify) occupied or controlled spaces. By their nature, both efficiency and reliability are critical attributes of chillers. Chillers typically range in thermal capacity from approximately 10 kW (3 ton) to exceeding 30 MW (8,500 ton), with a more common range of 300 kW (85 ton) to 14 MW (4,000 ton). Larger systems typically employ multiple chillers, with some installations exceeding 300 NW (85,000 ton) of cooling. Liquid-chilling systems cool water, brine, or other secondary coolant for air conditioning or refrigeration. The system may be either factory-assembled and wired or shipped in sections for erection in the field. The most frequent application is water chilling for air conditioning although brine cooling for low temperature refrigeration and chilling fluids in industrial processes are also common.
The basic components of a vapor-compression, liquid-chilling system include a compressor, liquid cooler (evaporator), condenser, compressor drive, liquid-refrigerant expansion or flow control device, and control center; it may also include a receiver, economizer, expansion turbine, and/or, subcooler. In addition, auxiliary components may be used, such as a lubricant cooler, lubricant separator, lubricant-return device, purge unit, lubricant pump, refrigerant transfer unit, refrigerant vents, and/or additional control valves.
Liquid (usually water) enters the cooler, where it is chilled by liquid refrigerant evaporating at a lower temperature. The refrigerant vaporizes and is drawn into the compressor, which increases the pressure and temperature of the gas so that it may be condensed at the higher temperature in the condenser. The condenser cooling medium is warmed in the process. The condensed liquid refrigerant then flows back to the evaporator through an expansion device. Some of the liquid refrigerant changes to vapor (flashes) as pressure drops between the condenser and the evaporator. Flashing cools the liquid to the saturated temperature at evaporator pressure. It produces no refrigeration in the cooler. The following modifications (sometimes combined for maximum effect) reduce flash gas and increase the net refrigeration per unit of power consumption.
Subcooling. Condensed refrigerant may be subcooled below its saturated condensing temperature in either the subcooler section of a water-cooled condenser or a separate heat exchanger. Subcooling reduces flashing and increases the refrigeration effect in the chiller.
Economizing. This process can occur either in a direct expansion (DX), an expansion turbine, or a flash system, in a DX system, the main liquid refrigerant is usually cooled in the shell of a shell-and-tube heat exchanger, at condensing pressure, from the saturated condensing temperature to within several degrees of the intermediate saturated temperature. Before cooling, a small portion of the liquid flashes and evaporates in the tube side of the heat exchanger to cool the main liquid flow. Although subcooled, the liquid is still at the condensing pressure.
An expansion turbine extracts rotating energy as a portion of the refrigerant vaporizes. As in the DX system, the remaining liquid is supplied to the cooler at intermediate pressure. In a flash system, the entire liquid flow is expanded to intermediate pressure in a vessel that supplies liquid to the cooler at saturated intermediate pressure; however, the liquid is at intermediate pressure.
Flash gas enters the compressor either at an intermediate stage of a multistage centrifugal compressor, at the intermediate stage of an integral two-stage reciprocating compressor, at an intermediate pressure port of a screw compressor, or at the inlet of a high-pressure stage on a multistage reciprocating or screw compressor.
Liquid Injection. Condensed liquid is throttled to the intermediate pressure and injected into the second-stage suction of the compressor to prevent excessively high discharge temperatures and, in the case of centrifugal machines, to reduce noise. For screw compressors, condensed liquid is injected into a port fixed at slightly below discharge pressure to provide lubricant cooling.
Basic System
An exemplary refrigeration cycle of a basic liquid chiller system is shown in FIG. 1. Chilled water enters the cooler at 54° F., for example, and leaves at 44° F. Condenser water leaves a cooling tower at 85° F., enters the condenser, and returns to the cooling tower near 95° F. Condensers may also be cooled by air or evaporation of water. This system, with a single compressor and one refrigerant circuit with a water-cooled condenser, is used extensively to chill water for air conditioning because it is relatively simple and compact. The compressor can be a reciprocating, scroll, screw, or centrifugal compressor. The preferred systems of the present invention are centrifugal liquid chiller systems.
Liquid chiller systems can also be used to fulfill heating requirement through heat recovery. Heat recovery is the process of capturing the heat that is normally rejected from the chiller condenser and using it for space heating, domestic water heating, or another process requirement. For water-cooled chillers, it can be accomplished either by operating at higher condensing temperatures and recovering heat from the water leaving the standard condenser, or by using a separate condenser. It can also be done by recovering heat from the refrigerant using a heat exchanger, preferably between the compressor and the condenser. Heat recovery in air-cooled chiller necessarily involves recovering heat from the refrigerant. The preferred heat recovery systems of the present invention are heat recovery centrifugal chillers.
A centrifugal compressor uses rotating elements to accelerate the refrigerant radially, and typically includes an impeller and diffuser housed in a casing. Centrifugal compressors usually take fluid in at an impeller eye, or central inlet of a circulating impeller, and accelerate it radially outwardly. Some static pressure rise occurs in the impeller, but most of the pressure rise occurs in the diffuser section of the casing, where velocity is converted to static pressure. Each impeller-diffuser set is a stage of the compressor. Centrifugal compressors are built with from 1 to 12 or more stages, depending on the final pressure desired and the volume of refrigerant to be handled. A compressor with more than one stage is called a multistage compressor.
Centrifugal compressors may use lubricating oil or may be oil-free. An example of oil-free compressors is those with magnetic bearings, where the rotor shaft is levitated between magnetic bearings and is preferably rotated using a direct drive motor, particularly a permanent magnet direct drive motor. Another example of oil-free compressors is those using hybrid bearing systems without oil, such as those using ceramic rolling elements.
The pressure ratio, or compression ratio, of a compressor is the ratio of absolute discharge pressure to the absolute inlet pressure. Pressure delivered by a centrifugal compressor is practically constant over a relatively wide range of capacities. Therefore, in order to maintain the centrifugal compressor performance while replacing the existing refrigerant, the pressure ratio when using the new refrigerant should be as close as possible to that when using the existing refrigerant.
Unlike a positive displacement compressor, a centrifugal compressor depends entirely on the centrifugal force of the high speed impeller to compress the vapor passing through the impeller. There is no positive displacement, but rather what is called dynamic-compression.
The pressure a centrifugal compressor can develop depends on the tip speed of the impeller. Tip speed is the speed of the impeller measured at its tip and is related to the diameter of the impeller and its revolutions per minute. The capacity of the centrifugal compressor is determined by the size of the passages through the impeller. This makes the size of the compressor more dependent on the pressure required than the capacity.
In order to maintain the centrifugal compressor performance while replacing the existing refrigerant, the predetermined impeller Mach number should be the same as that achieved by the existing refrigerant. Since impeller Mach number is dependent upon the acoustic velocity (speed of sound) of refrigerant, the performance of a compressor can more accurately be maintained by formulating a replacement refrigerant which has the same acoustical velocity as the original refrigerant, or which has an acoustical velocity which theoretically will provide the same impeller Mach number as the existing refrigerant.
An important consideration for compressors, especially when replacing an existing refrigerant with a new one, is the dimensionless specific speed, Ω, defined here as:
  Ω  =            ω      ⁢              V                            (                  Δ          ⁢                                          ⁢          h                )                    3        /        4            where ω is the angular velocity (rad/s), V is the volume flow rate (m3/s) and Δh is the ideal specific work (J/kg) per compressor stage, which can be approximated as:
      Δ    ⁢                  ⁢    h    =            h      2        -          h      1        -                  (                              s            2                    -                      s            1                          )            ⁢                                    T            2                    -                      T            1                                    ln          ⁡                      (                                          T                2                            /                              T                1                                      )                              where the subscripts 1 and 2 denotes the gas state at the compressor inlet and outlet respectively. H, s, and Tare respectively the specific enthalpy, specific entropy, and temperature. Compressors operate with the highest adiabatic efficiency, η, when the Ω has the optimum value for the design.
Because of its high speed operation, a centrifugal compressor is fundamentally a high volume, low pressure machine. A centrifugal compressor works best with a low pressure refrigerant, such as trichlorofluoromethane (CFC-11). When part of the chiller, particularly the evaporator, is operated with at a pressure level below ambient, the chiller is referred to as a negative pressure system. One of the benefits of a low pressure or negative pressure system is low leak rates. Refrigerant leaks are driven by pressure differentials, so lower pressures will result in lower leak rates than high pressure systems. Also, leaks in the system operating at below ambient pressure result in air being sucked into the equipment rather than refrigerant leaking out. While such operation requires a purge device to remove any air and moisture, monitoring the purge operation serves as a warning system for developing leaks.